(1) Field of the Invention
The present invention relates to a welded can having a novel laminate coating on the metal-exposed face of a seam and a process for the preparation thereof. More particularly, the present invention relates to a welded can excellent in the corrosion resistance, the appearance of the seam and the content-retaining property and a process for the preparation thereof.
(2) Description of the Prior Art
Since the bonded portion of a welded can is formed by welding metals at a high temperature under an elevated pressure, in case of a tinplate can, for example, a tin layer is isolated and the iron face becomes exposed to the outside. Furthermore, in case of a welded can of a steel sheet electrolytically treated with chromic acid (often called "TFS"), a chromium or chromium oxide layer of the portion to be bonded is removed by mechanical scraping before welding and the iron face is similarly exposed. Moreover, at a cut edge of a metal blank for a can, iron is exposed to the outside, and this cut edge is present on each of both the sides of the seam of the welded can. Since welding is carried out under high compression in the state where both the edges of a metal blank for a can are lapped together, molten iron spreads to the step portion of the seam to form an iron-exposed portion. When a canned food or the like is packed in such welded can, iron is dissolved out into the packed content from the iron-exposed portion, causing various fatal disadvantages. For example, the flavor of the packed content is degraded by iron dissolved out, and a pinhole can or hydrogen swollen can is formed by corrosion of the iron-exposed portion, with the result that the content-preserving property is reduced.
In addition to the above-mentioned problem of formation of an iron-exposed portion on the seam, the welded can involves a problem of reduction of the appearance characteristics such as blackening or browning of the appearance due to oxidation of iron in the seam portion (it is believed that triiron tetroxide is formed) and rusting of the iron-exposed face caused while an empty can or packed can is stored.
As means for overcoming these defects, a trial has been made to protect a metal-exposed face inclusive of a seam in a welded can with an organic resin coating formed by applying a varnish or the like. However, this trial is still insufficient for satisfactorily attaining the object of completely preventing iron from being dissolved out from the seam and preventing occurrence of corrosion of the seam. More specifically, a known organic resin coating material is poor in the adhesion to the metal-exposed face and even if a good adhesion to the metal-exposed face is obtained, the corrosion-preventive effect is insufficient or the processability of the coated portion is not good. For example, most of thermosetting resin varnishes fail to completely cover cut edges of seams, and coatings of these varnishes are poor in the processability and cracks or other defects are readily formed at the flanging process or seaming process. A coating composed of a tape of a thermoplastic resin is insufficient in the barrier property to corrosive components and no sufficient anti-corrosive effect can be obtained, and when the coated can is subjected to retort sterilization, the adhesion of the coating is drastically reduced.
As pointed out above, it is very difficult to form a coating excellent in the adhesion and corrosion resistance on a metal-exposed face inclusive of a seam in a welded can. In addition to this problem, a welded can involves the following problem. In the preparation of a welded can, the surface of a blank, except the edges to be formed into a seam, is preliminarily coated or printed with an organic resin varnish or ink, the coated or printed blank is cut into a predetermined size if necessary, the edges are lapped together and the blank is formed into a cylinder, and the lapped portion is welded to form a can body. Accordingly, an organic resin coating applied to the seam should have a good adhesion not only to the metal-exposed face but also to an edge portion of the coated or printed surface. Very few coatings show a good adhesion to both the metal-exposed face and the coated or printed surface, and none of organic resin coatings have a substantial adhesion to a coating or printing ink layer in which a lubricant has been incorporated so as to improve the running characteristic of the can-manufacturing blank or improve the scratch resistance of the coating or printing ink layer.